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When I double the width & length of input pair the phase margin changes from 60 to 40

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When I double the width & length of input pair the phase margin changes from 60 to 40

I design a opamp, when I double the width and length of input differential pair, the phase margin changed from 60 degree to 40 degree,
can anyone tell me the reason?
thank you
 

op-amp ac simulation with spectre

input pole
 

op-amp ac

You'd better specify the configuration of your OPamp.
Say,the folded cascode configuration,the Cgd of the input transistor will lower the 2nd pole,thus the PM.
 

opamp ac simulation

what's type of your opamp?
 

opamp ac simulation

you had better take other transistor into consideration,it may effect the character of other MOS
 

opamp ac simulation

the opamp is folded cascode, at first I think it is the cgd of input pair that degrade phase margin, but the cgd is 20f and gm=0.5u, this zero is about 4G and the unity gain bandwidth of opamp is 200M, so the cgd influence is small.
but beside this reason I cannot find other reason,
maybe leakage? my process is 90nm!

thabk you very much!

Added after 1 minutes:

sorry! the gm is 0.5m
 

opamp ac simulation

cap change, and pole change
 

opamp ac simulation

the dominant pole is at output,ya,input cgd's influence is small.make sure your dc bias makes every mosfet work at saturate region.and the unity gain frequency is 200M,do you need so large bandwidth?
 

opamp ac simulation

Hi qqic: I think you increase the w and l of the input transistor, then the input capacitance will increase, so the second pole i.e. gm/c will decrease, so the phase margin become worse!

regard!
 

opamp ac simulation

Hi all,
I know the reason,when input pair W*L increase, the cds increase quickly! from the DC operation point: cds=72f
cdg=-72.5f
cgd=-10.7f
cgs=-120f

I have 2 questions:
1> why the cdg is minus in DC simulation of spectre ?
2> why cgd is not the same to cdg, and cdg is much high?
3> why cds is much high? in my opinion cds should be very small?

thank you very much!
 

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